Base station and radio terminal

ABSTRACT

A user equipment, method, and apparatus for controlling user equipment receive, from a base station included in a network, a radio resource control (RRC) release message that causes the user equipment to transition to an RRC inactive state in which context information of the user equipment is maintained in the network. The RRC inactive state is a different state from an RRC connected state and an RRC idle state, and the RRC release message includes information indicating a value of a timer used for performing an RRC resume procedure. The timer is started upon receiving the RRC release massage, and the RRC resume procedure is performed when the timer expires.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/144,879 filed Sep. 27, 2018, which is a continuation application ofinternational application PCT/JP2017/011818, filed Mar. 23, 2017, whichclaims the benefit of U.S. Provisional Application No. 62/316,765, filedApr. 1, 2016, and U.S. Provisional Application No. 62/335,856, filed May13, 2016, the entire contents of which are incorporated herein byreference.

TECHNICAL FIELD

The present disclosure relates to a base station and a radio terminalused in a mobile communication system.

BACKGROUND ART

In recent years, with the spread of radio terminals such as smartphonescapable of executing a lot of applications, the frequency at which aradio terminal connects to a network and the frequency at which anetwork performs paging of a radio terminal are increasing.

Therefore, in a mobile communication system, network load accompanyingsignaling is increasing. In view of such a situation, techniques forreducing signaling are being studied in the 3rd Generation PartnershipProject (3GPP), which is the standardization project for mobilecommunication systems.

SUMMARY

A user equipment according to the present disclosure comprises areceiver configured to receive, from a base station included in anetwork, a radio resource control (RRC) release message that causes theuser equipment to transition to an RRC inactive state in which contextinformation of the user equipment is maintained in the network. The RRCinactive state is a different state from an RRC connected state and anRRC idle state, and the RRC release message includes informationindicating a value of a timer used for performing an RRC resumeprocedure. A controller configured to start the timer upon receiving theRRC release massage, and perform the RRC resume procedure when the timerexpires.

A method performed at a user equipment according to the presentdisclosure comprises receiving, from a base station included in anetwork, a radio resource control (RRC) release message that causes theuser equipment to transition to an RRC inactive state in which contextinformation of the user equipment is maintained in the network. The RRCinactive state is a different state from an RRC connected state and anRRC idle state, and the RRC release message includes informationindicating a value of a timer used for performing an RRC resumeprocedure. The method comprises starting the timer upon receiving theRRC release massage, and performing the RRC resume procedure when thetimer expires.

An apparatus for controlling a user equipment according to the presentdisclosure comprises a processor and a memory. The processor configuredto receive from a base station included in a network, a radio resourcecontrol (RRC) release message that causes the user equipment totransition to an RRC inactive state in which context information of theuser equipment is maintained in the network. The RRC inactive state is adifferent state from an RRC connected state and an RRC idle state, andwherein the RRC release message includes information indicating a valueof a timer used for performing an RRC resume procedure. The processor isconfigured to start the timer upon receiving the RRC release massage,and perform the RRC resume procedure when the timer expires.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an architecture of an LTE systemaccording to an embodiment.

FIG. 2 is a diagram illustrating an architecture of a UE (radioterminal) according to an embodiment.

FIG. 3 is a diagram illustrating an architecture of an eNB (basestation) according to an embodiment.

FIG. 4 is a diagram showing an architecture of a protocol stack of aradio interface according to an embodiment.

FIG. 5 is a diagram illustrating an architecture of a radio frameaccording to an embodiment.

FIG. 6 is a diagram illustrating an operation of a UE according to afirst embodiment.

FIG. 7 is a diagram illustrating an operation of a UE in a lightconnected state according to modification 2 of the first embodiment.

FIG. 8 is a diagram illustrating an operation of an eNB according to asecond embodiment.

FIG. 9 is a diagram illustrating an operation of a UE according to thesecond embodiment.

FIG. 10 is a diagram illustrating an operation of a UE according tomodification 1 of the second embodiment.

FIG. 11 is a diagram illustrating an operation of a UE according tomodification 2 of the second embodiment.

FIG. 12 is a diagram illustrating an operation according to modification1 of a third embodiment.

FIG. 13 is a diagram illustrating an operation according to a fifthembodiment.

DESCRIPTION OF EMBODIMENTS First Embodiment

(Architecture of Mobile Communication System)

Hereinafter, an architecture of a mobile communication system accordingto a first embodiment will be described. FIG. 1 is a diagramillustrating an architecture of a Long Term Evolution (LTE) system thatis the mobile communication system according to the first embodiment.The LTE system is a mobile communication system based on the 3GPPstandard.

As illustrated in FIG. 1, the LTE system includes a user equipment (UE)100, an evolved-UMTS terrestrial radio access network (E-UTRAN) 10, andan evolved packet core (EPC) 20.

The UE 100 corresponds to a radio terminal. The UE 100 is a mobilecommunication apparatus and performs radio communication with a cell(serving cell).

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes an evolved Node-B (eNB) 200. The eNB 200 corresponds to a basestation. The eNBs 200 are connected to each other via an X2 interface.

The eNB 200 manages one or more cells and performs radio communicationwith the UE 100 that has established connection to the cell. The eNB 200has a radio resource management (RRM) function, a user data(hereinafter, simply referred to as “data”) routing function, ameasurement control function for mobility control and scheduling, andthe like. The “cell” is used as the term indicating the smallest unit ofthe radio communication area and is also used as the term indicating thefunction of performing radio communication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes a mobilitymanagement entity (MME)/serving-gateway (S-GW) 300. The MME performsvarious types of mobility control or the like on the UE 100. The S-GWperforms data transfer control. The MME/S-GW 300 is connected to the eNB200 via an S1 interface.

FIG. 2 is a diagram illustrating the architecture of the UE 100 (radioterminal). As illustrated in FIG. 2, the UE 100 includes a receiver 110,a transmitter 120, and a controller 130.

The receiver 110 performs a variety of reception under the control ofthe controller 130. The receiver 110 includes an antenna and a receiver.The receiver converts a radio signal received by the antenna into abaseband signal (reception signal) and outputs the baseband signal tothe controller 130.

The transmitter 120 performs a variety of transmission under the controlof the controller 130. The transmitter 120 includes an antenna and atransmitter. The transmitter converts a baseband signal (transmissionsignal) output by the controller 130 into a radio signal and transmitsthe radio signal from the antenna.

The controller 130 performs a variety of control on the UE 100. Thecontroller 130 includes at least one processor and a memory. The memorystores a program executed by the processor and information used forprocessing by the processor. The processor includes a baseband processorthat performs modulation and demodulation, coding and decoding, and thelike of the baseband signal, and a central processing unit (CPU) thatperforms a variety of processes by executing a program stored in thememory. The processor performs a process to be described later.

FIG. 3 is a diagram illustrating the architecture of the eNB 200 (basestation). As illustrated in FIG. 3, the eNB 200 includes a transmitter210, a receiver 220, a controller 230, and a backhaul communication unit240.

The transmitter 210 performs a variety of transmission under the controlof the controller 230. The transmitter 210 includes an antenna and atransmitter. The transmitter converts a baseband signal (transmissionsignal) output by the controller 230 into a radio signal and transmitsthe radio signal from the antenna.

The receiver 220 performs a variety of reception under the control ofthe controller 230. The receiver 220 includes an antenna and a receiver.The receiver converts a radio signal received by the antenna into abaseband signal (reception signal) and outputs the baseband signal tothe controller 230.

The controller 230 performs a variety of control on the eNB 200. Thecontroller 230 includes at least one processor and a memory. The memorystores a program executed by the processor and information used forprocessing by the processor. The processor includes a baseband processorthat performs modulation and demodulation, coding and decoding, and thelike of the baseband signal, and a central processing unit (CPU) thatperforms a variety of processes by executing a program stored in thememory. The processor performs a process to be described later.

The backhaul communication unit 240 is connected to the neighbour eNB200 via an X2 interface and connected to the MME/S-GW 300 via an S1interface. The backhaul communication unit 240 is used for communicationperformed on the X2 interface, communication performed on the S1interface, and the like.

FIG. 4 is a diagram illustrating the architecture of the protocol stackof the radio interface in the LTE system. As illustrated in FIG. 4, aradio interface protocol is divided into a first layer to a third layerof an OSI reference model, and the first layer is a physical (PHY)layer. The second layer includes a medium access control (MAC) layer, aradio link control (RLC) layer, and a packet data convergence protocol(PDCP) layer. The third layer includes a radio resource control (RRC)layer.

The PHY layer performs coding and decoding, modulation and demodulation,antenna mapping and demapping, and resource mapping and demapping. Dataand control information are transmitted between the PHY layer of the UE100 and the PHY layer of the eNB 200 via a physical channel.

The MAC layer performs priority control of data, a retransmissionprocess by hybrid ARQ (HARQ), a random access procedure, and the like.Data and control information are transmitted between the MAC layer ofthe UE 100 and the MAC layer of the eNB 200 via a transport channel. TheMAC layer of the eNB 200 includes a scheduler that determines uplink anddownlink transport formats (transport block size, modulation and codingscheme (MCS)) and resource blocks allocated to the UE 100.

The RLC layer transmits data to the RLC layer on the receiving side byusing the functions of the MAC layer and the PHY layer. Data and controlinformation are transmitted between the RLC layer of the UE 100 and theRLC layer of the eNB 200 via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane that handles thecontrol information. A message (RRC message) for various configurationsis transmitted between the RRC layer of the UE 100 and the RRC layer ofthe eNB 200. The RRC layer controls logical channels, transportchannels, and physical channels in response to establishment,re-establishment, and release of radio bearers. If there is a connection(RRC connection) between the RRC of the UE 100 and the RRC of the eNB200, the UE 100 is in an RRC connected state; otherwise, the UE 100 isin an RRC idle state.

A non-access stratum (NAS) layer, which is located above the RRC layer,performs session management, mobility management, and the like.

FIG. 5 is a diagram illustrating the architecture of the radio frameused in the LTE system. As illustrated in FIG. 5, the radio frameincludes ten subframes arranged in a time direction. Each subframeincludes two slots arranged in the time direction. A length of eachsubframe is 1 ms, and a length of each slot is 0.5 ms. Each subframeincludes a plurality of resource blocks (RB) in a frequency directionand includes a plurality of symbols in the time direction. Each resourceblock includes a plurality of subcarriers in the frequency direction.One symbol and one subcarrier constitute one resource element (RE). Inaddition, among the radio resources (time and frequency resources)allocated to the UE 100, the frequency resource can be specified by theresource block and the time resource can be specified by the subframe(or slot).

In the downlink, a section of several symbols in the head of eachsubframe is a region that is mainly used as a physical downlink controlchannel (PDCCH) for transmitting downlink control information. Inaddition, the remaining portion of each subframe is a region that ismainly used as a physical downlink shared channel (PDSCH) fortransmitting downlink data. Basically, the eNB 200 transmits downlinkcontrol information (DCI) to the UE 100 by using the PDCCH and transmitsdownlink data to the UE 100 by using the PDSCH. The downlink controlinformation carried by the PDCCH includes uplink scheduling information,downlink scheduling information, and a TPC command. The uplinkscheduling information is scheduling information (UL grant) aboutallocation of uplink radio resources, and the downlink schedulinginformation is scheduling information about allocation of downlink radioresources. The TPC command is information instructing increase ordecrease of uplink transmission power. The eNB 200 includes a CRC bitscrambled with an identifier (RNTI: radio network temporary ID) of thedestination UE 100 in the downlink control information so as to identifythe UE 100 that is the transmission destination of the downlink controlinformation. Each UE 100 performs blind decoding on the PDCCH byperforming CRC check after descrambling with the RNTI of the UE in thedownlink control information that may be addressed to the UE, anddetects the downlink control information addressed to the UE. The PDSCHcarries downlink data by the downlink radio resource (resource block)indicated by the downlink scheduling information.

In the uplink, both end portions in the frequency direction in eachsubframe is a region that is mainly used as a physical uplink controlchannel (PUCCH) for transmitting uplink control information. Theremaining portion of each subframe is a region that is mainly used as aphysical uplink shared channel (PUSCH) for transmitting uplink data.

(Specific State)

Hereinafter, a specific state according to the first embodiment will bedescribed.

The specific state is a state in which signaling is reduced as comparedwith the RRC connected state while context information (UE context) ofthe UE 100 is maintained in the network. The UE context includesinformation about various configurations and capabilities for the UE100. The various configurations include a configuration of accessstratum (AS). The specific state includes a light connected state and asuspend state. It should be noted that the light connected state may bereferred to as a light connection state. In addition, the lightconnected (light connection) state may be referred to as a lightconnected (light connection) mode, and the suspend state may be referredto as a suspend mode.

The light connected state is a special RRC connected state in whichsignaling is reduced as compared with the RRC connected state. Forexample, the UE 100 in the light connected state is exempt fromtransmitting and receiving specific signaling with the network.Alternatively, the UE 100 in the light connected state reduces thefrequency of transmitting and receiving specific signaling with thenetwork. In addition, the light connected state may be a state in whichthe S1 connection to the UE 100 is maintained, or may be a state inwhich the RRC connection is released.

The suspend state is a special RRC idle state in which at least part ofthe UE context is maintained in the network. It should be noted that inthe case of the general RRC idle state, the UE context is discarded inthe network. The eNB 200 allocates a predetermined identifier (resumeID) when the UE 100 transitions to the suspend state. The UE 100notifies the eNB 200 of the predetermined identifier when the UE 100transitions from the suspend state to the RRC connected state. The eNB200 resumes the use of the UE context based on the predeterminedidentifier. If the UE 100 moves in the suspend state, the eNB 200 mayacquire the UE context from another eNB 200 connected via the X2interface. The suspend state is an RRC idle state and may be defined asa state in which the connection configuration or the like is held.Alternatively, the suspend state may be defined as an RRC suspend statethat is different from the RRC idle state and the RRC connected state.

The UE 100 can make a transition (that is, RRC connection setup) fromthe specific state to the RRC connected state with less signaling byusing the maintained UE context.

(Predetermined Area According to First Embodiment)

Hereinafter, the predetermined area according to the first embodimentwill be described. The mobile communication system according to thefirst embodiment introduces a new area unit that is different from acell and a tracking area. In the following, an area of such an area unitis referred to as a “predetermined area”. The predetermined area isapplied to the UE 100 in the specific state (a light connected state ora suspend state).

The predetermined area is formed by a group of cells or eNBs 200. Thepredetermined area according to the first embodiment is an area in whichthe network simultaneously performs paging transmission. Thepredetermined area is an area unit of a limited range as compared withthe tracking area. For example, the predetermined area is an area of apart of the tracking area. The predetermined area may be set within thesame tracking area, or may be set across a different tracking area.

By performing paging only in such a narrow area, it is possible toreduce the number of cells that perform paging transmission as comparedwith the case of performing paging in units of tracking areas.Therefore, it is possible to reduce signaling (paging). It should benoted that the paging transmission in the predetermined area unit may beperformed not on the initiative of the MME 300 (MME initiated) but onthe initiative of the eNB 200 (eNB initiated). Such paging may bereferred to as RAN-based paging.

(Operation According to First Embodiment)

In response to recognizing the movement of the UE 100 outside thepredetermined area formed by the group provided with the cells or eNBs200, the UE 100 according to the first embodiment transmits anotification indicating the movement to the network. The predeterminedarea is an area unit of a limited range rather than the tracking area.In the first embodiment, the predetermined area is applied while the UE100 is in the specific state. In other words, the UE 100 enables thetransmission of the notification indicating the movement only while theUE 100 is in the specific state. Therefore, the network (in particular,the MME 300) can grasp the predetermined area in which the UE 100 in thespecific state exists and can appropriately perform the paging withrespect to the predetermined area in which the UE 100 exists.

In the first embodiment, the UE 100 receives information indicating thepredetermined area from the network (the eNB 200 or the MME 300), andrecognizes the movement to the outside of the predetermined area basedon the received information. The information includes at least one of anidentifier (group ID) of the group forming the predetermined area, anidentifier list (cell ID list) of the cells included in the group, andan identifier list (eNB ID list) of the eNBs 200 included in the group.

FIG. 6 is a diagram illustrating the operation of the UE 100 accordingto the first embodiment.

As illustrated in FIG. 6, in step S11, the UE 100 receives, from the eNB200, an instruction (configuration information) to transition the UE 100to the specific state. For example, the eNB 200 transmits, to the UE100, the instruction (configuration information) to transition to thespecific state by using the UE-dedicated RRC signaling. The UE-dedicatedRRC signaling may be an RRC connection release. As a result, the UE 100transitions from the RRC connected state to the specific state.

In step S12, the UE 100 receives the information indicating thepredetermined area from the network. Step S12 may be performed at thesame time as step S11. There are a pattern in which the eNB 200transmits the information indicating the predetermined area and apattern in which the MME 300 transmits the information indicating thepredetermined area.

In the pattern in which the eNB 200 transmits the information indicatingthe predetermined area, the eNB 200 transmits, to the UE 100, at leastone of the group ID to which the eNB 200 (cell) belongs, the cell IDlist of the cells included in the group, and the eNB ID list of the eNBs200 included in the group by broadcast RRC signaling (for example, SIB)or UE-dedicated RRC signaling (for example, RRC connection release). TheeNB 200 may receive the group ID, the cell ID list, and the eNB ID listfrom the MME 300 through an MME configuration update or the like.

In the pattern in which the MME 300 transmits the information indicatingthe predetermined area, the MME 300 transmits, to the UE 100, the cellID list of the cells included in the group and the eNB ID list of theeNBs 200 included in the group by NAS signaling.

In step S13, the UE 100 recognizes the movement to the outside of thepredetermined area notified from the network based on the informationindicating the predetermined area. It should be noted that “movement tothe outside of the predetermined area” may be a movement to a cell or aneNB 200 not included in the predetermined area (group) notified from thenetwork. Alternatively, “movement to the outside of the predeterminedarea” may be a movement from one predetermined area to anotherpredetermined area.

If the cell ID list or the eNB ID list is notified, the UE 100determines whether the cell (or the eNB 200) in which the UE 100 existsis included in the list. If the cell (or the eNB 200) in which the UE100 exists is not included in the list, the UE 100 recognizes that theUE 100 has moved to the outside of the predetermined area. Otherwise,the UE 100 recognizes that the UE 100 is within the predetermined area.

If the group ID is notified from the eNB 200, when the UE 100 moves fromone cell (or one eNB) to another cell (or another eNB), the UE 100determines whether the group ID notified from the one cell (or the oneeNB) is the same as the group ID notified from the another cell (or theanother eNB). In the case of different group IDs, the UE 100 recognizesthat the UE 100 has moved to the outside of the predetermined area.Otherwise, the UE 100 recognizes that the UE 100 is within thepredetermined area.

When the UE 100 recognizes that the UE 100 has moved to the outside ofthe predetermined area, the UE 100 transmits a notification indicatingthe movement to the network in step S14. The notification indicating themovement may include at least one of the group ID, the cell ID, and theeNB ID. These IDs are a movement source ID and/or a movement destinationID. There are a pattern in which the notification indicating themovement is transmitted to the MME 300 and a pattern in which thenotification indicating the movement is transmitted to the eNB 200.

In the pattern in which the notification indicating the movement istransmitted to the MME 300, the UE 100 transmits the notification to theMME 300 by NAS signaling (NAS message). Based on the notification, theMME 300 determines a cell (or eNB 200) to be a transmission destinationof the paging of the UE 100. It should be noted that the eNB 200 maydecode the NAS message, may read the information of the UE 100 (the IDof the UE 100 in the cell), and may store the information.

In the pattern in which the notification indicating the movement istransmitted to the eNB 200, the UE 100 transmits the notification to theeNB 200 by RRC signaling (or MAC control element). The eNB 200 mayforward the received notification to the MME 300. At that time, the eNB200 may further notify the MME 300 of the cell ID (or the group ID orthe like).

In the pattern in which the notification indicating the movement istransmitted to the eNB 200, the UE 100 transmits the notification to theeNB 200 (cell) of the movement source or the eNB 200 (cell) of themovement destination. If the notification is transmitted to the eNB 200(cell) of the movement source, the UE 100 notifies the cell (eNB 200)belonging to the predetermined area before leaving the predeterminedarea. The notification may include the cell ID (and/or the eNB ID, thegroup ID) of the movement destination cell. The eNB 200 may transmit theUE context of the UE 100 to the cell belonging to the new area. If thenotification is transmitted to the eNB 200 (cell) of the movementdestination, the UE 100 notifies the (new) cell (eNB 200) not belongingto the predetermined area after leaving the predetermined area. Thenotification may include the cell ID (and/or the eNB ID, the group ID)of the movement source cell. The eNB 200 may make a request for the UE100 context to the movement source cell (movement source eNB).

In the pattern in which the notification indicating the movement istransmitted to the eNB 200, the UE 100 may perform notification afterestablishing the RRC connection, or may perform notification withoutestablishing the RRC connection.

In the pattern in which the notification is performed withoutestablishing the RRC connection, the UE 100 in the light connected statetransmits the notification along the scheme of the light connected stateif uplink synchronization is established (that is, if timing advance canbe acquired). On the other hand, if the uplink synchronization is notestablished (that is, if the timing advance cannot be acquired), arandom access procedure may be performed and the notification may beperformed during the random access procedure.

In the pattern in which the notification is performed withoutestablishing the RRC connection, the UE 100 in the suspend state mayperform the random access procedure and perform the notification duringthe random access procedure.

Here, the operation of performing the notification during the randomaccess procedure will be described. The UE 100 transmits a random accesspreamble (Msg1) to the eNB 200 and receives a random access response(Msg2) from the eNB 200. Next, the UE 100 performs the notification tothe eNB 200 instead of transmitting an RRC connection request or an RRCconnection reestablishment request in Msg3, or in these messages. Thenotification includes the identifier of the UE 100 (IMSI, S-TMSI, resumeID, or the like). Then, the random access procedure in Msg4 isterminated without establishing the RRC connection. That is, the eNB 200does not transmit the RRC connection setup. It may be notified as aninformation element of RRC connection request and RRC connectionreestablishment.

(Modification 1 of First Embodiment)

In the first embodiment, the cell (or the eNB 200) to which the UE 100transmits the notification may be limited. For example, the eNB 200transmits an identifier (permission information) indicating whether theUE 100 is allowed to perform the notification by broadcast RRC signaling(SIB). The UE 100 transmits the notification only to the cell (or theeNB 200) that transmits the permission information.

(Modification 2 of First Embodiment)

In the above-described first embodiment, an example in which the UE 100in the light connected state performs the random access procedure if theuplink synchronization is not established (that is, if the timingadvance cannot be acquired) has been described. The UE 100 can establishthe uplink synchronization by performing the random access procedure. Inthis manner, the UE 100 in the light connected state can maintaindownlink synchronization, but cannot always maintain the uplinksynchronization. It should be noted that the UE 100 determines that theuplink is asynchronous when a time alignment timer (TAT) expires.

In the general LTE system, the UE 100 that is not synchronized with theuplink (for example, the UE 100 in the RRC idle state) starts the randomaccess procedure in response to generation of uplink data. On the otherhand, it is preferable that the light connected state can return to theRRC connected state more quickly. In addition, if the UE 100 receivesdownlink data, there is a high possibility that uplink transmission (forexample, ACK/NACK transmission) will occur after that.

According to modification 2 of the first embodiment, if the UE 100 is inthe light connected stat, the UE 100 starts the random access procedurefor the eNB 200 in response to the UE's reception of the downlink datafrom the eNB 200. Here, the UE 100 starts the random access procedure inresponse to the reception of the downlink data even before the uplinkdata is generated. In this manner, it is possible to return to theconnected state more quickly by starting the random access procedureeven before the uplink data is generated.

FIG. 7 is a diagram illustrating the operation of the UE 100 in thelight connected state according to modification 2 of the firstembodiment. As illustrated in FIG. 7, in step S101, the UE 100 in thelight connected state receives downlink data from the eNB 200. In stepS102, the UE 100 checks whether uplink synchronization is established.If the uplink synchronization is not established (step S102: NO), the UE100 starts the random access procedure in step S103. As a result, the UE100 transitions to the connected state.

Second Embodiment

In the second embodiment, differences from the first embodiment will bemainly described below.

A predetermined area according to the second embodiment will bedescribed. The predetermined area is formed by a group of cells or eNBs200. The predetermined area according to the second embodiment isapplied to the UE 100 in a specific state (a light connected state or asuspend state). In addition, the predetermined area according to thesecond embodiment is an area unit in which the network can maintaincontext information. The predetermined area may be formed by a pluralityof eNBs 200 mutually connected via an X2 interface.

Even if the UE 100 that has transitioned to the specific state withinthe predetermined area moves to another cell (other eNB 200) within thepredetermined area, the UE 100 can perform the RRC connection setup withless signaling. On the other hand, when the UE 100 that has transitionedto the specific state within the predetermined area moves to the outsideof the predetermined area, the UE context is not maintained, and it isnecessary to newly create the UE context. Therefore, continuing thespecific state is not appropriate.

The eNB 200 according to the second embodiment may transition the UE 100to the specific state and may transmit the context information of the UE100 to another eNB that forms the predetermined area together with theeNB 200 (that is, another eNB in the same group as the eNB 200). Asdescribed above, the eNB 200 according to the second embodiment assumesthat the UE 100 moves to the outside of the cell (the eNB 200) andpreviously shares the context information with another eNB. Therefore,when the UE 100 performs the RRC connection setup, it is possible toreduce the time for acquiring the UE context between the eNBs, and thusthe RRC connection setup can be promptly performed.

In addition, a first operation or a second operation for avoidingcontinuing to hold the UE context permanently in the eNB may be added.

In the first operation, if the (own/another) eNB performs the RRCconnection with the UE 100 (for example, if the RRC connection isresumed), information (for example, UE context release) indicating thatthe pre-shared UE context can be discarded may be notified to anothereNBs in the same group. The discardable information may includeinformation (for example, resume ID and UE X2AP ID) for specifying theUE context. The eNB that has received the discardable information maydiscard the corresponding UE context.

In the second operation, when the eNB 200 transmits the UE context toanother eNB, the eNB 200 may further transmit information (timer value,time, or the like) indicating the validity period of the UE context. TheeNB that has received the UE context may discard the UE context inresponse to the expiration of the validity period.

According to the second embodiment, if the UE 100 is in the specificstate, the UE 100 stops the specific state in response to recognizingthe movement of the UE 100 to the outside of the predetermined area. Inother words, the UE 100 validates the specific state only within thepredetermined area. In this case, the UE 100 may transition from thespecific state to the RRC connected state, or may transition from thespecific state to the RRC idle state. If the UE 100 transitions from thespecific state to the RRC connected state, the UE 100 may transmit theRRC connection request to the eNB 200. In addition, the UE 100 maydiscard the configuration related to the specific state. Therefore,occurrence of an unexpected error can be prevented even if moving to theoutside of the predetermined area.

FIG. 8 is a diagram illustrating the operation of the eNB 200 accordingto the second embodiment. As illustrated in FIG. 8, in step S21, the eNB200 transitions the UE 100 to the specific state. For example, the eNB200 transmits, to the UE 100, the instruction (configurationinformation) to transition to the specific state by using theUE-dedicated RRC signaling. The UE-dedicated RRC signaling may be an RRCconnection release. As a result, the UE 100 transitions from the RRCconnected state to the specific state. In step S22, the eNB 200transmits the UE context of the UE 100 to another eNB in the same groupas the eNB 200. For example, the eNB 200 transmits the UE context on theX2 interface. The eNB 200 may transmit the UE context to all the eNBs(except for the eNB 200) in the same group as the eNB 200. The anothereNB that has received the UE context stores the received UE context anduses the UE context when the UE 100 moves to the eNB.

FIG. 9 is a diagram illustrating the operation of the UE 100 accordingto the second embodiment. As illustrated in FIG. 9, the operations ofsteps S31 to S33 are the same as those of the first embodiment. When themovement of the UE 100 to the outside of the predetermined area at thetime of the transition to the specific state is recognized, the UE 100stops the specific state in step S34.

(Modification 1 of Second Embodiment)

In the above-described second embodiment, an example in which the eNB200 performs the configuration of the light connected state (theinstruction to transition from the RRC connected state to the lightconnected state) individually for the UE has been described. That is,the UE 100 receives the configuration of the specific state from the eNB200 by dedicated signaling, and transitions from the RRC connected stateto the specific state in response to the reception of the configuration.

In addition, in the above-described second embodiment, the UE 100discards the configuration when the UE 100 transitions from the lightconnected state to the RRC connected state. That is, the UE 100 discardsthe configuration in response to the transition from the specific stateto the RRC connected state.

If the configuration for the light connected state remains whenreturning to the RRC connected state, it is unclear whether the UE 100transitions to the light connected state or transitions to the RRC idlestate when the UE 100 makes a state transition from the RRC connectedstate. In particular, if the transition to the light connected state ismade by the RRC connection release, such a problem becomes conspicuous.Therefore, the problem can be avoided by discarding the configuration.

In modification 1 of the second embodiment, such an operation will bedescribed in more detail. FIG. 10 is a diagram illustrating theoperation of the UE 100 according to modification 1 of the secondembodiment.

As illustrated in FIG. 10, in step S201, the UE 100 receives theconfiguration of the light connected state from the eNB 200 byUE-dedicated signaling. Here, an RRC connection release or RRCconnection reconfiguration message is assumed as the UE-dedicatedsignaling.

The configuration of the light connected state may include an identifierinstructing to transition to the light connected state. For example, theeNB 200 includes an information element (IE) such as “light connectedsetup” in the RRC connection release or RRC connection reconfigurationmessage.

Alternatively, the transition to the light connected state may beimplicitly instructed by including the following configuration in theRRC connection release or RRC connection reconfiguration message.

-   -   DRX configuration for light connected state. Such a DRX        configuration will be described in a fourth embodiment.    -   Timer value related to state transition. The UE 100 starts the        timer when the transition to the light connected state is        instructed, and maintains the light connected state while the        timer is in operation. Then, the UE 100 transitions to the RRC        connected state in response to the expiration of the timer.    -   Cell list. The configuration information corresponds to the        “information indicating the predetermined area” described in the        first embodiment.

In step S202, the UE 100 transitions from the RRC connected state to thelight connected state according to the configuration for the lightconnected state.

In step S203, the UE 100 transitions to the RRC connected state again inresponse to satisfaction of the condition for the transition to the RRCconnected state in the light connected state.

In step S204, the UE 100 discards the configuration for the lightconnected state.

(Modification 2 of Second Embodiment)

In the above-described second embodiment, the UE 100 transitions to theRRC connected state or the RRC idle state in response to stopping thelight connected state (specific state). Here, it has been assumed thatwhether to transition to the RRC connected state or transition to theRRC idle state is entrusted or predefined to the autonomousdetermination of the UE 100.

In modification 2 of the second embodiment, the eNB 200 can configurewhether to transition to the RRC connected state or transition to theRRC idle state. Therefore, the behavior of the UE 100 can be controlledby the eNB 200.

According to modification 2 of the second embodiment, if the UE 100 isin the light connected state, the UE 100 stops the specific state inresponse to satisfaction of a predetermined condition. The predeterminedcondition may be that the timer related to the state transition hasexpired, or may be that the UE 100 has left the predetermined area.

The UE 100 receives, from the eNB 200, information for configuring theoperation of the UE 100 when the light connected state is stopped. Whenthe light connected state is stopped, the UE 100 performs the operationconfigured from the eNB 200.

FIG. 11 is a diagram illustrating the operation of the UE 100 accordingto modification 2 of the second embodiment.

As illustrated in FIG. 11, in step S211, the UE 100 receives theconfiguration of the light connected state from the eNB 200. Theconfiguration of the light connected state is included in, for example,the RRC connection release message. The configuration of the lightconnected state includes an indicator specifying whether to transitionto the RRC connected state or transition to the RRC idle state after thelight connected state.

In step S212, the UE 100 transitions to the light connected state.

In step S213, the UE 100 transitions to the RRC connected state or theRRC idle state according to the configuration from the eNB 200.

Third Embodiment

In the third embodiment, differences from the first and secondembodiments will be described below. In the third embodiment, the lightconnected state is mainly assumed as the specific state.

According to the third embodiment, if the UE 100 is in the specificstate (light connected state), the eNB 200 transmits downlink data tothe UE 100 without transmitting a paging message to the UE 100. In otherwords, the eNB 200 according to the third embodiment performs downlinktransmission without performing paging to the UE 100 in the specificstate (light connected state). In addition, according to the thirdembodiment, if the UE 100 is in the specific state (light connectedstate), the UE 100 receives the downlink data from the eNB 200 withoutreceiving the paging message from the eNB 200. Therefore, according tothe third embodiment, it is possible to reduce signaling (paging) withrespect to the UE 100 in the light connected state.

In the third embodiment, in order to receive the downlink data, the UE100 in the light connected state monitors the PDCCH at the same timingas the operation in the RRC connected state. For example, the UE 100monitors the PDCCH at the on timing (on duration) of DRX (discontinuousreception) in the RRC connected state. If there is PDSCH allocation inthe PDCCH, the UE 100 receives the downlink data by the PDSCH.

The eNB 200 receives the downlink data for the UE 100 from the S-GW 300,performs PDCCH transmission to the UE 100 at the timing when the UE 100monitors the PDCCH, and transmits the downlink data to the UE 100through the PDSCH. At this time, the eNB 200 does not transmit thepaging message to the UE 100.

The MME 300 recognizes the UE 100 in the light connected state as an ECMconnected state and does not transmit, to the eNB 200, the S1 pagingmessage corresponding to the UE 100.

(Modification 1 of Third Embodiment)

The eNB 200 according to modification 1 of the third embodimenttransmits downlink data to the UE 100 in the light connected state bymulti-cell transmission that simultaneously performs transmission from aplurality of cells including the cell. Here, the eNB 200 transmits thedownlink data to the UE 100 without transmitting the paging message tothe UE 100.

In this manner, since the downlink data is transmitted in units of areas(predetermined areas) including the plurality of cells, the network doesnot need to grasp the cell in which the UE 100 exists. Therefore, the UE100 does not transmit position information to the network whenever theUE 100 moves the cell, and may notify the network when moving thepredetermined area. It should be noted that the multi-cell transmissionmay be performed only within the area (predetermined area) in which theUE 100 can move transparently to the eNB 200.

FIG. 12 is a diagram illustrating the operation according tomodification 1 of the third embodiment. FIG. 12 illustrates an examplein which a predetermined area #1 is formed by a plurality of cells(cells #1, #2, . . . ) and the respective cells are managed by differenteNBs 200. However, one eNB 200 may manage a plurality of cells. The UE100 in the light connected state exists in the cell #2 in thepredetermined area #1. It should be noted that the network grasps thatthe UE 100 exists in the predetermined area #1, but does not grasp thatthe UE 100 exists in the cell #2.

As illustrated in FIG. 12, in step S301, the EPC 20 (S-GW) transmits, tothe eNB 200-1, downlink data (DL data 1) destined for the UE 100.

In step S302, the eNB 200-1 forwards, to the eNB 200-2, at least a part(DL data 2) of the DL data 1 received from the EPC 20 (S-GW). The DLdata 2 may be the same amount of data as a normal paging message (thatis, a small amount of data). In addition, the eNB 200-1 and the eNB200-2 may share the configuration for multi-cell transmission. Theconfiguration may include the C-RNTI of the UE 100, or may include theresume ID of the UE 100.

Instead of forwarding the DL data 2 between the eNBs 200, the EPC 20(S-GW) may multicast the DL data 2 to all the eNBs 200 in thepredetermined area #1.

In step S303, the eNB 200-1 and the eNB 200-2 transmit the DL data 2 byusing the multi-cell transmission by the cell #1 and the cell #2. TheeNB 200-1 and the eNB 200-2 may use the C-RNTI or the resume ID as thedestination ID of the DL data 2. If the resume ID is used, scramble ofPDCCH may be performed by resume ID, or PDSCH transmission may beperformed without PDCCH. The transmission resource of the DL data 2(that is, the PDSCH resource) may be the narrowband resource describedin the fourth embodiment. The transmission MCS of the DL data 2 may be afixed value, or may be designated in the configuration of the lightconnected state.

In step S304, the UE 100 that has received the DL data 2 transmits anACK corresponding to the DL data 2 to the cell #2 that is a servingcell. As described above, if the uplink synchronization is notestablished, the UE 100 may perform the random access procedure. The UE100 may transmit the ACK to the cell #2 in the random access procedure.For example, the UE 100 includes the ACK in the random access procedureMsg1 (random access preamble) or Msg3 (RRC connection request message).

In step S305, the eNB 200-2 (cell #2) that has received the ACK from theUE 100 determines that the UE 100 exists in the cell (cell #2). In otherwords, the network handles the ACK as the paging response.

On the other hand, in step S306, the eNB 200-1 (cell #1) that does notreceive the ACK from the UE 100 determines that the UE 100 does notexist in the cell (cell #1).

It should be noted that it is preferable that the time period duringwhich each eNB 200 waits for ACK is a time length with allowance to someextent in consideration of the possibility that the UE 100 performs therandom access procedure.

In the operation of FIG. 12, the eNB 200-1 determines that the UE 100does not exist in the cell (cell #1) in response to not receiving theACK from the UE 100 within the waiting time of the ACK. However, insteadof such a method, notification may be performed from the eNB 200-2 tothe eNB 200-1, and the eNB 200-2 may perform the determination based onthe notification. For example, the eNB 200-2 notifies the EPC 20 thatthe ACK has been received. Further, the EPC 20 notifies another eNB 200(eNB 200-1) in the predetermined area #1. Alternatively, the eNB 200-2may directly notify another eNB 200 (eNB 200-1) in the predeterminedarea #1, in addition to such notification via the EPC 20.

(Modification 2 of Third Embodiment)

In the above-described third embodiment and modification 1 thereof, theeNB 200 transmits the downlink data to the UE 100 in the light connectedstate without transmitting the paging message. On the other hand, asdescribed in the first embodiment, there may be an option of performingRAN-based paging that is paging performed on the initiative of the eNB200 (eNB initiated).

The eNB 200 according to the third embodiment notifies the UE 100 ofinformation indicating which of the first paging mode and the secondpaging mode is to be applied. The first paging mode is a mode oftransmitting the downlink data without transmitting the paging message(see the third embodiment). The second paging method is a mode (that is,RAN-based paging) in which the downlink data is transmitted aftertransmitting the paging message on the initiative of the eNB 200. The UE100 applies the first paging mode or the second paging mode according tothe configuration from the eNB 200. The configuration may be notified bybroadcast signaling (SIB) or UE-dedicated signaling (dedicatedsignaling). The eNB 200 may determine which of the first paging mode andthe second paging mode is to be applied based on the type of the cell(for example, macro/small) and the moving state (for example, speed orthe like) of the UE 100.

Fourth Embodiment

In the fourth embodiment, differences from the first to thirdembodiments will be described below. The fourth embodiment is anembodiment related to intermittent reception (DRX) for the lightconnected state.

The UE 100 according to the fourth embodiment performs not onlyintermittent reception on the time axis but also intermittent receptionin the frequency direction (that is, narrowband reception) as the DRXapplied at the time of light connected. The narrowband reception meansto monitor frequencies in the range narrower than the range offrequencies to be monitored by the existing DRX. In the existing DRX,the UE 100 monitors the entire system bandwidth (the bandwidth of eachcell) for all the active cells (primary cell and at least one secondarycell) used by the UE 100. On the other hand, in the DRX applied at thetime of light connected, the UE 100 monitors only some of the activecells used by the UE 100 and/or monitors only a part of the systembandwidth.

The UE 100 according to the fourth embodiment performs a firstintermittent reception (existing DRX) if the UE 100 is in the RRCconnected state or the RRC idle state, and performs a secondintermittent reception (DRX for the light connected state) if the UE 100is in the specific state. The second intermittent reception is anintermittent reception in which the range of frequencies to be monitoredis limited as compared with the first intermittent reception.

The operation and parameters of the DRX for the light connected state inthe time axis direction are the same as those of the existing DRX.

On the other hand, the operation and parameters of the DRX for the lightconnected state in the frequency axis direction are parameters that aredifferent from those of the existing DRX. The parameters of the DRX forthe light connected state in the frequency axis direction may includeinformation about the resource block (PRB: physical resource block). Theresource block information includes at least one of a resource blocknumber at the start/end positions of the frequency range to bemonitored, and the number of resource blocks corresponding to thebandwidth of the frequency range to be monitored. The parameters of theDRX for the light connected state in the frequency axis direction mayinclude a carrier number (ARFCN: absolute radio frequency channelnumber). The parameters of the DRX for the light connected state in thefrequency axis direction are designated from the network (eNB 200) tothe UE 100.

The UE 100 monitors the PDCCH in the set frequency domain in the settime interval. The UE 100 may receive the narrowband PDCCH (M-PDCCH).Such an operation may be performed only if the parameters of the DRX forthe light connected state in the frequency axis direction are configuredto the UE 100.

In this manner, since it is only necessary to monitor the frequencyrange narrower than in the conventional case by performing thenarrowband reception in the DRX for the light connected state, the powerconsumption of the UE 100 can be reduced. In addition, since it ispossible to allocate different frequency resources (carriers and/orresource blocks) for each UE 100, load distribution is expected.

It should be noted that the parameters of the DRX for the lightconnected state in the frequency axis direction may be shared by thecells (eNBs 200) within the predetermined area (see modification 1 ofthe third embodiment). The parameters may be shared in advance, or maybe shared at the time of occurrence of communication (context fetch orthe like).

Alternatively, the parameters of the DRX for the light connected statein the frequency axis direction may be invalid if the UE 100 leaves thecell after the UE 100 transitions to the light connected state in acertain cell. In this case, the UE 100 shifts to a full band monitorwhen the UE 100 transitions to another cell. The UE 100 may validateonly the DRX in the time direction in the separate cell, or mayinvalidate the DRX in the time direction.

Fifth Embodiment

In the fifth embodiment, differences from the first to fourthembodiments will be described below.

The fifth embodiment is an embodiment that makes it possible totransition to the RRC idle state on the initiative of the UE 100 (UEinitiated) in the light connected state. Therefore, the UE 100 candetermine transition to the RRC idle state according to the situation ofthe UE 100 (for example, the situation of the application executed bythe UE 100).

However, when the UE 100 freely permits transition to the RRC idlestate, it is difficult for the network to grasp the state of the UE 100.Assuming that the light connected state is ECM-connected from theviewpoint of the core network (MME), it is preferable that the networkcan grasp the state of the UE 100.

If the UE 100 according to the fifth embodiment is in the lightconnected state, the UE 100 transmits, to the eNB 200, informationnotifying or requesting that the UE 100 transitions to the RRC idlestate. In the case of using the request, the UE 100 may transition tothe RRC idle state only when the UE 100 receives an acknowledgment fromthe eNB 200.

With such notification (or request), the network can grasp the state ofthe UE 100. Due to this, the network can determine whether to use MMEinitiated paging (paging for RRC idle state) or RAN-based paging (pagingfor light connected state) in the case of calling the UE 100.

FIG. 13 is a diagram illustrating the operation according to the fifthembodiment. In the initial state of FIG. 13, the UE 100 is in the lightconnected state.

As illustrated in FIG. 13, in step S501, the UE 100 determines whetherto transition to the RRC idle state. For example, based on the state ofthe application layer or the like, the UE 100 determines to transitionto the RRC idle state if the possibility of data communicationdisappears (for example, if the session of the upper layer isdisconnected).

In step S502, the UE 100 transmits, to the eNB 200, informationnotifying or requesting that the UE 100 transitions to the RRC idlestate. Here, it is assumed that the UE 100 transmits the notification byusing Msg1 or Msg3 of the random access procedure.

If Msg1 is used, the random access preamble is transmitted by using asignal sequence associated with the identifier of the UE 100 (forexample, resume ID, C-RNTI, S-TMSI, IMSI). The eNB 200 identifies the UE100 from the sequence. Alternatively, the random access preamble istransmitted by using the radio resource (time/frequency resource)associated with the identifier of the UE 100. The eNB 200 identifies theUE 100 from the radio resource. The association information may benotified (informed) to the UE 100 in advance. The eNB 200 transmits Msg2(random access response) to the UE 100 as the response to Msg1.

If Msg3 is used, the notification is stored in the RRC message (RRCconnection request). The eNB 200 transmits Msg4 to the UE 100 as theresponse to Msg3.

In step S503, the eNB 200 may indicate, to the UE 100, whether totransition to the RRC idle state by Msg4 (OK or NG).

In the case of OK, the eNB 200 may transmit a connection establishmentrejection message (RRC connection reject) to the UE 100 as Msg4.Therefore, the UE 100 stops the random access procedure and transitionsto the RRC idle state without transitioning to the RRC connected state(step S504). Alternatively, the eNB 200 may notify the UE 100 of thetransition to the RRC idle state through Msg2.

In this manner, since the UE 100 performs the notification in the courseof the random access procedure without completing the random accessprocedure, it is possible to prevent the UE 100 from transitioning tothe RRC connected state only for the notification. Therefore, signalingor the like can be reduced. It should be noted that the same operationcan be performed in the detach request of the NAS, but if the detachrequest is used, the UE 100 must transition to the RRC connected state.

On the other hand, in the case of NG, the eNB 200 may cause the UE 100to transition to the RRC connected state. The eNB 200 transmits aconnection establishment permission message (RRC connection setup) tothe UE 100 as Msg4, and the UE 100 transitions to the RRC connectedstate. Alternatively, the eNB 200 may notify the UE 100 of the rejectionof the transition to the RRC idle state through Msg2.

In step S505, the eNB 200 may release the RRC connection (step S505) andtransmit a UE context release request to the EPC 20 (MME) (step S506).

Other Embodiments

The specific state may be valid only during a period in which the timerconfigured to the UE 100 is in operation. In this case, the UE 100 stopsthe specific state in response to the expiration of the timer.Alternatively, the specific state may be valid only during a period inwhich the UE 100 is within a predetermined frequency. For example, theUE 100 that has received the instruction of the specific state (lightconnection) in a certain cell ends the specific state in response to themovement to a cell having a frequency different from a frequency towhich the cell belongs.

In the above-described embodiment, an example in which the predeterminedarea is applied only to the UE 100 in the specific state has beendescribed. However, the predetermined area can also be applied to the UE100 in the idle state.

In the above-described embodiment, the UE 100 may make an RRC connection(request) in response to the fact that the UE 100 has moved to theoutside of the predetermined area. In this case, after entering the RRCconnected state, the UE 100 waits for an instruction or notificationfrom the eNB 200 (for example, acquisition of new area information,transition to the specific state, or the like). It should be noted thatthe UE 100 may configure the cause (establishment cause) of the RRCconnection request (RRC connection request) to a value indicating“leaving to the outside of the area”.

The present disclosure is not limited to the case in which theabove-described embodiments are separately and independently performed,but two or more embodiments may be performed in combination. Forexample, a part of configurations according to one embodiment may beadded to other embodiments. Alternatively, a part of configurationsaccording to one embodiment may be replaced with a part ofconfigurations of other embodiments.

In the above-described embodiment, the LTE system has been exemplifiedas the mobile communication system. However, the present disclosure isnot limited to the LTE system. The present disclosure may be applied tosystems other than the LTE system. For example, the embodiment may beapplied to the 5th generation communication system (5G system). In the5G system, an inactive state (inactive mode) is being studied as a newRRC state, and the light connection state in the embodiment may bereplaced with an inactive state. When the embodiment is applied to the5G system, the RAN paging may be replaced with the RAN notification andthe RAN paging area may be replaced with the RAN notification area.

(Additional Note 1)

Introduction

The new work item on Signalling reduction to enable light connection forLTE was agreed. According to the approved WID, the study phase isplanned before the normative work and the objective in the initial phaseis as follows.

In the study phase, investigate potential solutions for the followingaspects, taking into account both UE mobility and traffic pattern:

Signalling reduction due to handover, considering UE centric mobility,e.g. cell (re)-selection.

Signaling reduction due to Paging, considering limiting the Pagingtransmission within a more limited area.

Signalling reduction to CN over S1 interface due to mobility and statetransitions by hiding them from CN.

UE context storage and retrieval along with UE mobility across differenteNBs.

Necessity of a new RAN based state.

From the perspective of RAN2, the Signalling reduction due to Paging isidentified for the initial discussion of the study. In this additionalnote, possible issues in the current paging mechanism are discussed.

Discussion

The paging message is used to inform the UEs of the availability of MTcalls, the notification of SI update, ETWS, CMAS, and EAB parametersmodification, and the trigger of load redistribution. It was reportedthat the paging message makes up 26.8% of overall RRC signalling load,as statistics in practical LTE networks. Considering all IEs other thanthe paging information are defined with 1-bit encoding type such asENUMERATED {true} in Paging, the paging information, i.e.,pagingRecordList, is the dominant cause of the signalling load due to apaging message. So, it's effective to consider how the actual paginginformation contents can be reduced for MT calls, e.g., due to S1PAGING. With such a reduction, it would be possible to reduce the numberof transmitted bits within a paging message and the option for the NW tovary the number of the paging transmissions.

RAN2 should prioritize the study on the reduction of paging informationconveyed within paging messages, i.e., the paging record lists.

RRC States

The easiest way to significantly reduce the number of paging messageswould be to force all the UEs in a tracking area to stay in Connected,but it's also the wrong approach from the UE's power consumption pointof view. So, this should not be used as the basis for signallingreduction of the paging message.

UEs should not be kept in RRC Connected, i.e., Rel-13 connected modejust to reduce the number of pages.

It also necessary to evaluate the RRC connection suspend/resumeprocedures, i.e., the UP solution for NB-IoT, although it's still anon-going discussion in RAN2. Based on the agreements so far, it may beassumed that the RRC connection resume is used for the UE to transitionfrom IDLE to Connected, i.e., the UE stays in IDLE when the RRCconnection is suspended. Thus, it is necessary for the NW to page the UEin the Suspend mode for MT calls. This means the gains from pagingcontent reduction cannot be realized if the size of PagingUE-Identity isnot significantly different, e.g., difference in the length between theresume ID and S-TMSI/IMSI is small. It is also necessary to consider thenumber of additional paging transmissions that may be needed as a resultof the Suspend mode.

Even if the UE is in RRC Suspend mode, the NW will still need to pagethe UE for MT calls.

UE Mobility

Before Rel-13, paging messages were transmitted in all cells within atracking area, regardless of whether the target UE(s) is actuallylocated in the cell transmitting the message. In Rel-13, the pagingoptimizations were introduced by RAN3 and SA2 for the signallingreduction on Uu as well as S1, e.g., the Recommended Cells for Paging IEwithin S1 PAGING. These Rel-13 mechanisms are efficient especially forUEs with low mobility, e.g., MTC UEs, but there may be a room forfurther optimizations considering UEs with normal mobility, e.g.,smartphones. For example, even when the MME determines from/informs theeNB of the recommended eNB/cell within S1 PAGING based on its knowledgeof Information On Recommended Cells And ENBs at transition to ECM IDLE,the UE upon this MT call may have already moved outside of therecommended eNB/cell. This would result in missed pages, and theresources used for the pages are wasted. It may be avoided if the eNBknows the UE's location, e.g., by means of a notification from the UEupon cell reselection. So, it's worth discussing how the eNB knows thelocation of the UE even in IDLE, to prevent unnecessary pages.

RAN2 should discuss if it's useful for the NW to know the location ofUEs in IDLE.

(Additional Note 2)

1. Introduction

RAN2 starts their discussion on Signalling reduction to enable lightconnection for LTE. It was extensively discussed on the definition ofLight Connection and the gain of paging enhancements, and finally acouple of working assumptions were agreed as follows.

-   -   Work assumption to study the paging enhancement is “S1        connection of a UE lightly connected is kept and active, in        order to hide the mobility and state transitions from CN”    -   Work assumption: Light connected UE can be addressed only by the        trigger of paging initiated by eNB or MME.

In this additional note, the details of paging enhancements and LightConnection are discussed, under the working assumptions.

2. Discussion

The paging message is used to inform the UEs of the availability of MTcalls, the notification of SI update, ETWS, CMAS, and EAB parametersmodification, and the trigger of load redistribution. It was reportedthat the paging message makes up 26.8% of overall RRC signalling load,as statistics in practical LTE networks. Considering all IEs other thanthe paging information are defined with 1-bit encoding type such asENUMERATED {true} in Paging, the paging information, i.e.,pagingRecordList, is the dominant cause of the signalling load due to apaging message. So, it's effective to consider how the actual paginginformation contents can be reduced for MT calls, e.g., due to S1PAGING. With such a reduction, it would be possible to reduce the numberof transmitted bits within a paging message and the option for the NW tovary the number of the paging transmissions.

Proposal 1: RAN2 should prioritize the study on the reduction of paginginformation conveyed within paging messages, i.e., the paging recordlists.

2.1. RRC States and Modes

2.1.1. RRC Connected State

The easiest way to significantly reduce the number of paging messageswould be to force all the UEs in a tracking area to stay in Connected,but it's also the wrong approach from the UE's power consumption pointof view. So, this should not be used as the basis for signallingreduction of the paging message.

Proposal 2: UEs should not be kept in RRC Connected, i.e., Rel-13connected mode, just to reduce the number of pages.

2.1.2. RRC Suspended Mode

It also necessary to evaluate the RRC connection suspend/resumeprocedures, i.e., the UP solution for NB-IoT, from the paging point ofview. Based on the agreements so far, it's assumed that the RRCconnection resume is used for the UE to transition from IDLE toConnected, i.e., the UE stays in IDLE when the RRC connection issuspended. For example, it's agreed that “UE in connected mode with ASsecurity activated can be released into idle mode or idle mode with thesuspend indication”. In other words, the RRC Suspended mode is just aspecial condition of RRC IDLE.

Observation 1: RRC Suspended mode is a special condition of the UE inIDLE.

Thus, it is necessary for the NW to page the UE in the Suspend mode forMT calls, as similar with the UE in IDLE. This means the gains frompaging content reduction cannot be realized if the size ofPagingUE-Identity is not significantly different, e.g., difference inthe length between the resume ID and S-TMSI/IMSI is small. It is alsonecessary to consider the number of additional paging transmissions thatmay be needed compared to Connected state, as a result of the Suspendmode.

Observation 2 Even if the UE is in RRC Suspend mode, the NW will stillneed to page the UE for MT calls.

2.1.3. RRC Light Connected Mode

As discussed above, LTE has two RRC states, i.e., Connected and IDLE,and a special condition of IDLE, i.e., Suspended mode. When the LightConnected is introduced, it should be discussed whether it's defined asa new RRC state or a special condition of the existing RRC state. If anew RRC state is defined, it's foreseen to define the transitionsbetween three states, whole UE behaviours under the new state, thecorresponding control messages and so on, wherein excessivestandardization efforts will be necessary. From the simplicity point ofview, RAN2 should stick to the current modelling with two RRC states,and therefore, the Light Connected should be defined as a specialcondition of Connected. This modelling could be well matched with theworking assumption “S1 connection of a UE lightly connected is kept andactive, in order to hide the mobility and state transitions from CN”,which may assume ECM-Connected from the perspective of CN.

Proposal 3: Light Connected mode should be defined as a specialcondition of RRC Connected, not a new state, even if it's introduced.

2.2. Paging Enhancements

2.2.1. Issue in Paging Optimizations Due to UE Mobility

Before Rel-13, paging messages were transmitted in all cells within atracking area, regardless of whether the target UE(s) is actuallylocated in the cell transmitting the message. In Rel-13, the pagingoptimizations were introduced by RAN3 and SA2 for the signallingreduction on Uu as well as S1, e.g., the Recommended Cells for Paging IEwithin S1 PAGING. These Rel-13 mechanisms are efficient especially forUEs with low mobility, e.g., MTC UEs, but there may be a room forfurther optimizations considering UEs with normal mobility, e.g.,smartphones. For example, even when the MME determines from/informs theeNB of the recommended eNB/cell within S1 PAGING based on its knowledgeof Information On Recommended Cells And ENBs at transition to ECM IDLE,the UE upon this MT call may have already moved outside of therecommended eNB/cell. This would result in missed pages, and theresources used for the pages are wasted.

Observation 3: Paging optimizations introduced in Rel-13, e.g., theRecommended Cells for Paging IE within S1 PAGING, may work effectivelyonly for stationary or low mobility UEs.

2.2.2. Expected Gains with Paging Enhancements

The working assumption mentions that “S1 connection of a UE lightlyconnected is kept and active, in order to hide the mobility and statetransitions from CN”, which also implies the UE is in ECM-Connected. Inthis case, the MME does not need to initiate the paging procedure when aDL data comes for the UE. So, at least from S1 signalling point of view,the signalling reduction will be achieved with a solution under theworking assumption.

Observation 4: Signalling reduction of S1 PAGING could be achieved withLight Connected mode.

The various solutions with RAN-level paging mechanism were proposed. Oneof the benefits in the solutions is to limit the paging area. It indeedcontributes to reduce number of paging messages in a whole network, ifthe RAN-level paging area is set to a subset of a tracking area. Similargain may be achieved with a NW implementation today, e.g., the trackingarea is configured with smaller region. But it has been pointed out thatsuch a NW implementation will cause excessive Tracking Area Updates fromUEs, whereby the overall signalling will likely increase.

Observation 5: Although the number of paging messages can be reduced ifa smaller paging area is configured, excessive Tracking Area Updates maynot be preventable.

From the observations above, the introduction of paging enhancementswill offer much benefit and the baseline solution is for theintroduction of RAN-level paging. Additionally, in the New RAT SI, manycompanies proposed to consider some kind of RAN-based paging mechanismto track UEs with low activity, in order to optimize the signalling andthe performance for longer battery life. These obviously imply thecurrent CN-based paging mechanism has some room for improvement and anarea that needs to be considered in this WI. So, RAN2 should discussfurther details of RAN-level paging mechanism.

Proposal 4: RAN2 should consider the details of RAN-level pagingconcept.

2.3. RAN-Level Paging Mechanism

2.3.1. Paging Message

If Proposal 4 is acceptable, the eNB may notice the necessity of pagingupon arrival of DL data for the UE, instead of S1 PAGING. The workingassumption mentions “Light connected UE can be addressed only by thetrigger of paging initiated by eNB or MME”, which suggests some sort ofpaging message is sent to the UE. From the U-plane data flow point ofview, the difference at this point between the legacy paging(MME-initiated) and the new paging (eNB-initiated) is whether the DLdata is still in the S-GW or already in the eNB, i.e., the routing inthe CN is already done. So, it would be worth considering whether thepaging message is really necessary in this case, although it's naturalway that the eNB sends the (RAN-level) paging message to the UE. One ofthe other possibilities may aim to eliminate any paging messages overUu. For example, the eNB sends the DL data immediately, instead of apage to the UE. If the DL data volume can be managed efficiently and iscomparable to the amount of data needed for paging messages, thedifferences may be minimal from a spectral efficiency perspective. Thedetails may be related to how to define the Light Connected mode,including mobility (UE-based or NW-based), thus should be FFS.

Proposal 5: RAN2 should discuss whether the UE should be paged with apaging message (as similar to today) or the direct DL data transmission.

2.3.2. Paging Area

It could also discussed that any transmission to page the UE isperformed in a specific area, like the existing tracking area, and it'sassumed as a group of cells, i.e., the paging area. It's straightforward to introduce such a concept to minimize paging failure. Thepaging area may be defined by availability of X2 connectivity for the UEcontext fetch, mobility state of the UE, balancing to spectralefficiency and so on, while it may be almost up to NW implementation.

Proposal 6: RAN2 should introduce the paging area, which consists of agroup of cells to send a transmission to page the UE.

It may be assumed that the UE mobility is transparent to the eNB as longas the UE is within the paging area, On the other hand, it should bediscussed how the UE behaves when it moves outside of the paging area.It's a reliable way to inform the serving cell when it happens, sinceit's quite similar to the existing Tracking Area Update. It's FFSwhether the information is provided before the reselection of an outsidecell or after it happens.

Proposal 7: RAN2 should discuss whether the UE should inform the servingcell when it moves outside of the paging area.

2.4. Alternative Consideration

As an alternative, the issue discussed in section 2.2.1 may be avoidedif the eNB knows the UE's location, e.g., by means of a notificationfrom the UE upon cell reselection. So, it's worth discussing how the eNBknows the location of the UE even in IDLE, to prevent unnecessary pages.It may be solved by the combination of the paging area, i.e., Proposal6, and the information, i.e., Proposal 7.

Proposal 8: As an alternative to the RAN-level paging concept, RAN2should discuss if it's useful for the NW to know the location of UEs inIDLE, when Rel-13 paging optimization is performed.

(Additional Note 3)

1. Introduction

In this additional note, the general issues in Light Connection otherthan paging aspects are identified.

2. Discussion

The working assumptions agreed in the last meeting uses the terminology“a UE lightly connected” or “Light connected UE”, which is one stepahead of the WI title, i.e., Light Connection is somewhat related toUE's condition. The objective of WID also states “Necessity of a new RANbased state” and “the solution can consider reusing the Suspend/Resumeprocedure”. So, it's one of important aspects how to model the lightconnection, e.g., reusing the RRC Suspend/Resume concept or introducinga new RRC state.

Observation 1: Modelling of Light Connection may be discussed togetherwith paging enhancements.

Regardless of the modelling of Light Connection for paging, thefollowing aspects could be discussed as agreed in the WID.

The solution shall apply for both mobile-originated andmobile-terminated data.

The solution shall enable the UE power consumption to be comparable tothat one in RRC_IDLE.

In general, the features to be adopted for Light Connection should becompared against the existing features as discussed in the followingsections.

2.1. General Features

2.1.1. Data Transmission and Reception Aspects (DL/UL/SL)

If the Light Connection is introduced, it needs to be clarified whetherthe Light connected UE is required to perform data transmission andreception, i.e., downlink (DL), uplink (UL) and sidelink (SL). In theexisting IDLE mode, only SL is allowed with “softly” controlled by theeNB, i.e., Type 1 or Mode 2 transmission within the configurationprovided in SIB18/19, while DL and UL needs the control signalling inadvance, e.g., Paging, RACH and/or RRC Connection Request. In Connectedmode, DL and UL are “tightly” controlled by the eNB, i.e., DL assignmentand UL grant, while SL may require tight control, i.e., Type 2B or Mode1 transmission by dedicated resource or SL grant, depending on the eNB'spreference.

Proposal 1: RAN2 should discuss on what the UE behaviour is for datatransmission/reception over Downlink, Uplink and Sidelink in LightConnection.

2.1.2. Measurements and Reporting Aspects (CSI/RLM/RRM)

UEs in Connected perform the various types of measurements, i.e., CSImeasurement, RLM measurement and RRM measurement, as well as measurementfeedback/reporting. On the other hand, UEs in IDLE only perform RRMmeasurement for cell reselection without reporting, i.e., UE-basedmobility. With Light Connection, it is necessity to discuss which ofthese measurements and feedback/reporting needs to be supported, whichshould depend on whether Light Connection is more like CONNECTED orIDLE.

Proposal 2: RAN2 should discuss which measurement and reportingmechanisms, CSI feedback, RLM/RRM measurements, needs to be supportedfor Light Connection.

2.1.3. Activation and Deactivation Aspects (SCell, SPS)

The SCell(s) could be configured for Carrier Aggregation and DualConnectivity, and these are activated or deactivated by e.g., the MACControl Element. Also, SPS is configured for efficient delivery of e.g.,VoLTE, and it's activated by PDCCH scrambled with SPS-RNTI. In thecurrent specification, SCell(s)/SPS are de-configured when the UEtransitions to IDLE, and it's reconfigured as needed when/after the UEtransitions back to Connected. With Light Connection, it's alsonecessary to define whether SCell(s) and SPS are deactivated or evende-configured.

Proposal 3: RAN2 should discuss whether SCell(s) and SPS is deactivatedor de-configured, when the UE transitions from CONN to Light Connection.

2.1.4. Assistance Information from UE Aspects

The current RRC supports many indications from the UE to assist with theeNB's functional control of various mechanisms, i.e., ProximityIndication, In-device Coexistence Indication, UE Assistance Information(Power Preference Indication), MBMS Interest Indication, MBMS CountingResponse and Sidelink UE Information. In PHY layer, the soundingreference signal, SRS, is also used e.g., to estimate UL channel. InLight Connection, it is necessary to discuss if any of the indicationsshould still be supported by the UE.

Proposal 4: RAN2 should discuss whether the UE in Light Connectioncontinues the eNB assistance with Proximity Indication, In-deviceCoexistence Indication, UE Assistance Information, MBMS InterestIndication, MBMS Counting Response, Sidelink UE Information, and SRS.

2.2. Other Features

2.2.1. Dual Connectivity

In addition to SCell(s) discussed in section 2.1.3, it may be definedwhether PSCell should be de-configured when the UE moves into LightConnection. If the PSCell is still applicable in Light Connection, itshould also be discussed whether SCG Failure Indication needs to bedeclared.

Proposal 5: RAN2 should discuss whether PSCell is de-configured, whenthe UE moves into Light Connection.

2.2.2. WLAN Interworking/Aggregation (RALWI, RCLWI, LWA, LWIP)

In Rel-12 and Rel-13, a couple of mechanisms for WLAN Interworking weredeveloped, i.e. RAN-assisted and RAN-controlled LTE-WLAN Interworkingsolutions, RALWI/RCLWI. The LWI mechanisms allow the NW to control UE inConnected its traffic steering to/from WLAN by the dedicated RANassistance parameter or the steering command After the UE transitions toIDLE, the configuration, which was set when the UE was in Connected, isstill applicable during T350 is running. In addition for RALWI, SIB17may provide the RAN assistance parameters and controls the UE in IDLE aswell as in Connected. It should be further discussed how the UE performsRALWI/RCLWI under Light Connection.

Proposal 6: RAN2 should discuss how the UE performs RALWI/RCLWI duringLight Connection.

In Rel-13, a set of WLAN aggregation solutions were specified, i.e.,LTE-WLAN Aggregation (LWA) and one using IPsec tunnel (LWIP). The LWAbearer is routed over WLAN link and terminated at the eNB and the UE.Considering the Light Connection over Uu, it also needs to be clarifiedhow the LWA configuration and LWA bearer(s) are handled when the UE isin Light Connection.

Proposal 7: RAN2 should discuss how the LWA bearer(s) is handled inLight Connection.

2.2.3. MDT

The minimization of drive test, MDT, was introduced in Rel-10 andcontinuously enhanced onward. The MDT consists of two modes, i.e.,Logged MDT for IDLE/Connected modes and Immediate MDT for Connectedmode. The MDT measurement log is sent over the measurement reportingwhen the UE is in Connected, wherein the logging continued even if theUE is in IDLE, in case of Logged MDT. For Light Connection, it has to bediscussed which MDT mode is supported.

Proposal 8: RAN2 should discuss which MDT mode is supported for UEs inLight Connection.

2.2.4. MCLD

The multi-carrier load distribution, MCLD, supports the tworedistribution mechanisms, the continuous redistribution scheme (CRS)and the one-shot scheme (OSS). These mechanisms is provided theredistribution parameter in SIB3/SIB5, and the UE in IDLE selects theredistribution target according to its IMSI upon either T360 expiry(CRS) or reception of the redistribution indication within the paging(OSS). For Light Connection, the load redistribution mechanisms may beapplicable since current assumption is that the UE should performUE-based mobility.

Proposal 9: RAN2 should discuss whether load redistribution is supportedin Light Connection.

In addition to above, it may also have to discuss whether anyenhancements are needed in the current features due to the introductionof Light Connection, e.g., some special handling such as an implicitdeactivation/de-configuration.

Proposal 10: Additionally, RAN2 should also discuss whether anyenhancements are necessary for the existing features due to theintroduction of Light Connection.

The invention claimed is:
 1. A user equipment comprising: a receiverconfigured to receive, from a base station included in a network, aradio resource control (RRC) release message that causes the userequipment to transition to an RRC inactive state in which contextinformation of the user equipment is maintained in the network, whereinthe RRC inactive state is a different state from an RRC connected stateand an RRC idle state, and wherein the RRC release message includesinformation indicating a value of a timer used for performing an RRCresume procedure; and a controller configured to start the timer uponreceiving the RRC release massage, and perform the RRC resume procedurewhen the timer expires.
 2. The user equipment according to claim 1,wherein the controller is configured to start the timer and enter theRRC inactive state upon receiving the RRC release massage.
 3. A methodperformed at a user equipment, the method comprising: receiving, from abase station included in a network, a radio resource control (RRC)release message that causes the user equipment to transition to an RRCinactive state in which context information of the user equipment ismaintained in the network, wherein the RRC inactive state is a differentstate from an RRC connected state and an RRC idle state, and wherein theRRC release message includes information indicating a value of a timerused for performing an RRC resume procedure, starting the timer uponreceiving the RRC release massage, and performing the RRC resumeprocedure when the timer expires.
 4. An apparatus for controlling a userequipment, the apparatus comprising a processor and a memory, theprocessor configured to receive from a base station included in anetwork, a radio resource control (RRC) release message that causes theuser equipment to transition to an RRC inactive state in which contextinformation of the user equipment is maintained in the network, whereinthe RRC inactive state is a different state from an RRC connected stateand an RRC idle state, and wherein the RRC release message includesinformation indicating a value of a timer used for performing an RRCresume procedure, start the timer upon receiving the RRC releasemassage, and perform the RRC resume procedure when the timer expires.